Monolithic catalysts for cleaning exhaust gases, for example by oxidation of CO or hydrocarbons to CO2 and water or by reduction of NOx with ammonia or urea to N2 and water, or for decomposing urea or its thermal decomposition product, isocyanic acid, to ammonia and CO2, have been known for some time.
As a rule, these catalysts are constructed by covering a channelled monolithic support material (honeycomb) with a coating (washcoat) having a large surface area, for example consisting of Al2O3, SiO2, SnO2 or TiO2, and applying to these metal-oxide surfaces the actually catalytically active metals or metal compounds, such as for example noble metals or transition metal oxides, and optionally additional promoter compounds/dopants. However, there are also applications in which the metal oxide coatings alone are catalytically active. A typical application example for this is the hydrolysis of isocyanic acid to ammonia with TiO2-coated honeycombs.
The honeycombs consist either of a so-called honeycomb body which can be composed of a honeycomb casing and a support inserted therein, in particular a partially structured and coiled metal foil, or consist entirely of a ceramic shaped body. The honeycombs are substantially pervaded by channels running parallel to the axis of the honeycombs.
For example, materials such as cordierite, steatite, Duranit® or silicon carbide, or shaped bodies consisting of silicon dioxide, aluminum oxides, aluminates or also metals and metal alloys are used as support material for honeycombs consisting of ceramic shaped bodies. The use of metals and metal alloys makes it possible in particular to produce honeycomb bodies with complex structures, such as for example honeycombs with open channel structures or with complex mixed systems.
As a rule, a honeycomb-shaped catalyst is produced by applying a washcoat (WC) to the channel walls (coating), followed by drying, then calcining at high temperatures for solidification, and finally surface engineering of the washcoat. Then the catalytically active components are applied to the washcoat by impregnation steps, usually from the aqueous solutions of their precursors. However, it is also possible to apply the active components or their precursor compounds directly with the coating process. This is carried out as a rule by impregnating the powder which is used to produce the washcoat with active components or their precursor compounds, whereupon drying and calcining take place.
The coating of a honeycomb body with the inorganic materials with a large surface area is possible using various methods. As a rule, a suspension of the inorganic support oxide in water is first produced, optionally with the addition of additives, such as inorganic binders, surfactants, catalytic active components, pore formers, rheology adjuvants and other additives, whereupon the honeycomb body is filled with this so-called coating suspension by an immersion, suction or pumping process.
Methods are described with which only the precisely calculated quantity of coating suspension (also called washcoat suspension) that is to remain in the honeycomb is introduced, and this quantity is distributed as evenly as possible on the channel walls.
In other methods, an excess of coating suspension is introduced into the honeycomb (e.g. flow-coating the honeycomb), whereupon a removal procedure is carried out, with which excess coating suspension is discharged. The removal can be carried out for example by blowing out by means of an air flow or by extraction by suction.
Several of these method variants are cited and described in U.S. Pat. No. 6,627,257. The removal of the excess coating suspension from the honeycomb by means of a centrifuge unit is described for example in GB 1504060.
Honeycombs with high cell densities, as well as honeycombs with perforated channels with complex structure (open structures) require special coating methods, in particular as blowing out the excess coating suspension with air is no longer possible with open channel structures. With such honeycombs, therefore, centrifugation is used to remove the excess coating suspension.
The use of vibrations during the application of the washcoat is described in DE 101 14 328 A1. Thus, on the one hand, the flowability of the coating suspension is to be improved and, on the other hand, the washcoat application is to be as even as possible.
A method is known from United States patent application publication 2008/0200328 for removing the excess of a washcoat suspension used to coat a honeycomb body having channels. In this case, the excess is removed with the help of a porous support that is applied to the end face of the honeycomb body on which the excess is to be discharged (discharge end). The average pore diameter of the porous support used is identical to or smaller than the average diameter of the channels of the honeycomb body.
A disadvantage of the known coating suspensions is that a sedimentation of the solid constituents at different speeds often results during the coating process, i.e. in particular as long as the coating suspension has not yet been dried. Within the framework of the present invention, it has been found that the sedimentation speeds of the solid, particulate constituents of the coating suspension differ and affect the coating process. Regardless of the coating method used, the result of the sedimentation process is that the constituents that sediment quickly sink more quickly in the coating suspension and are the first to be deposited on the catalyst substrate. An unevenly coated product is therefore obtained due to the change in the solids content of the coating suspension that takes place during the coating procedure. A further problem is that, to avoid a predominant sedimentation of the constituents that sediment more quickly in a container in which the coating suspension is present, constant stirring is needed.
The different sedimentation speeds of the particles therefore result in an inhomogenization of the suspension, with the result that the quantity ratios of the particles change in relation to one another. In addition, due to the uneven deposition of the constituents, the quantity ratios of the differently sized particles in the liquid coating suspension and in the deposited coating also change dependent on time. Thus, the ratio of the different particles to each other in the coating suspension then differs from the ratio of the different particles to each other in the already deposited coating. In particular if an excess of the coating suspension is again extracted from the catalyst substrate by suction, neither the composition of the applied coating nor the composition of the excess of the coating suspension extracted by suction corresponds to the composition of the solid constituents of the coating suspension originally used.
In addition, the different sedimentation speeds can lead to a coating that consists of individual layers in which the deposited constituents are present, separated from each other in layers according to their different sedimentation speeds. This is extremely disadvantageous for the properties of the catalyst, as above all the outermost layer, which is in contact with the reaction gases, is responsible for the catalytic activity and a homogeneous distribution of the constituents of the coating suspension in the outer layer of the catalyst is most advantageous.
In order to show this sedimentation effect, within the framework of the present invention a coating suspension which comprised aluminum oxide that was impregnated with platinum (black oxide) and a zeolite white in color was applied to a catalyst substrate and dried overnight. The resultant coating had two layers, one lower, white layer and one upper, black layer. This experiment shows that the zeolite has a higher sedimentation speed than the aluminum oxide and these different sedimentation speeds of the particles lead to a layered deposition of the two constituents of the coating suspension.